Today, liquid crystal display devices are used for various applications. In a general liquid crystal display device, one color display pixel includes three pixels for displaying red, green and blue, which are three primary colors of light, and thus color display is provided.
However, a conventional liquid crystal display device has a problem that a range of colors which can be displayed (referred to as a “color reproduction range”) is narrow. In order to broaden the color reproduction range of a liquid crystal display device, a technique of increasing the number of primary colors used for display has been proposed.
For example, Patent Document 1 discloses, as shown in FIG. 14, a liquid crystal display device 800 in which one color display pixel CP includes four pixels which are a red pixel R for display red, a green pixel G for displaying green, a blue pixel B for displaying blue, and a yellow pixel Y for displaying yellow. In the liquid crystal display device 800, color display is provided by mixing the four primary colors of red, green, blue and yellow displayed by the four pixels R, G, B and Y.
When display is provided by use of four or more primary colors, the color reproduction range can be broadened as compared with the range realized by the conventional liquid crystal display device which provides display by use of three primary colors. In this specification, a liquid crystal display device which provides display by use of four or more primary colors will be referred to as a “multiple primary color liquid crystal display device”, and a liquid crystal display device which provides display by use of three primary colors will be referred to as a “three primary color liquid crystal display device”.
Patent Document 2 discloses, as shown in FIG. 15, a liquid crystal display device 900 in which one color display pixel CP includes four pixels which are a red pixel R, a green pixel G, a blue pixel B, and a white pixel W for displaying white. In the liquid crystal display device 900, the color reproduction range cannot be broadened because the added pixel is the white pixel W, but the display luminance can be raised.
However, when the liquid crystal display device 800 shown in FIG. 14 or the liquid crystal display device 900 shown in FIG. 15, in which one color display pixel CP includes an even number of pixels, is driven by dot inversion driving, a phenomenon called “horizontal shadow” occurs, which reduces the display quality. Dot inversion driving is a technique for suppressing generation of display flicker. By dot inversion driving, the polarity of the applied voltage is inverted pixel by pixel.
FIG. 16 shows a polarity of a voltage applied to each pixel in the case where a three primary color liquid crystal display device is driven by dot inversion driving. FIG. 17 and FIG. 18 respectively show a polarity of a voltage applied to each pixel in the case where the liquid crystal display devices 800 and 900 are driven by dot inversion driving.
As shown in FIG. 16, in the three primary color liquid crystal display device, the polarities of the voltages applied to the pixels of the same color are inverted in a row direction. For example, in the first, third and fifth pixel rows in FIG. 16, the polarities of the voltages applied to the red pixels R are positive (+), negative (−) and positive (+) from left to right. The polarities of the voltages applied to the green pixels G are negative (−), positive (+) and negative (−) from left to right. The polarities of the voltages applied to the blue pixels B are positive (+), negative (−) and positive (+) from left to right.
By contrast, in the liquid crystal display devices 800 and 900, one color display pixel CP includes even number of (four) pixels. Therefore, as shown in FIG. 17 and FIG. 18, the polarities of all the voltages applied to the pixels of the same color in each pixel row are the same. For example, in the first, third and fifth pixel rows in FIG. 17, the polarities of the voltages applied to the red pixels R and the yellow pixels Y are all positive (+). The polarities of the voltages applied to the green pixels G and the blue pixels B are all negative (−). In the first, third and fifth pixel rows in FIG. 18, the polarities of the voltages applied to the red pixels R and the blue pixels B are all positive (+). The polarities of the voltages applied to the green pixels G and the white pixels W are all negative (−).
In the case where the polarities of all the voltages applied to the pixels of the same color arrayed in the row direction are the same, when a window pattern is displayed with a single color, a horizontal shadow is generated. Hereinafter, a cause of the horizontal shadow will be described with reference to FIG. 19.
As shown in FIG. 19(a), when a high luminance window WD is displayed with a single color so as to be surrounded by a low luminance background BG, a horizontal shadow SD having a higher luminance than that of the display which would be provided in a proper state may be generated to the right and to the left of the window WD.
FIG. 19(b) is an equivalent circuit of an area corresponding to two pixels in a general liquid crystal display device. As shown in FIG. 19(b), each of the pixels includes a thin film transistor (TFT) 14. A gate electrode, a source electrode and a drain electrode of the TFT 14 are electrically connected to a scanning line 12, a signal line 13 and a pixel electrode 11, respectively.
The pixel electrode 11, a counter electrode 21 provided so as to face the pixel electrode 11, and a liquid crystal layer located between the pixel electrode 11 and the counter electrode 21 form a liquid crystal capacitance CLC. A storage capacitance electrode 17 electrically connected to the pixel electrode 11, a storage capacitance counter electrode 15a provided so as to face the storage capacitance electrode 17, and a dielectric layer (insulating film) located between the storage capacitance electrode 17 and the storage capacitance counter electrode 15a form a storage capacitance Ccs.
The storage capacitance counter electrode 15a is electrically connected to a storage capacitance line 15, and is supplied with a storage capacitance counter voltage (CS voltage). FIG. 19(c) and FIG. 19(d) show changes of the CS voltage and the gate voltage in accordance with the time passage. In FIG. 19(c) and FIG. 19(d), the polarity of a write voltage (gray scale voltage to be supplied to the pixel electrode 11 via the signal line 13) is different from each other.
When the gate voltage is put into an ON state and thus the pixel starts to be charged, the potential of the pixel electrode 11 (drain voltage) is changed. At this point, as shown in FIG. 19(c) and FIG. 19(d), a ripple voltage is superimposed on the CS voltage via a parasitic capacitance between the drain and the CS. As can be seen from a comparison between FIG. 19(c) and FIG. 19(d), the polarity of the ripple voltage is inverted in accordance with the polarity of the write voltage.
The ripple voltage superimposed on the CS voltage is attenuated as time passes. In the case where the amplitude of the write voltage is small, namely, in a pixel for displaying the background BG, when the gate voltage is put into an OFF state, the ripple voltage becomes almost zero. By contrast, in the case where the amplitude of the write voltage is large, namely, in a pixel for displaying the window WD, the ripple voltage is higher than that of the pixel for displaying the background BG. Therefore, as shown in FIG. 19(c) and FIG. 19(d), when the gate voltage is put into an OFF state, the ripple voltage superimposed on the CS voltage is not attenuated sufficiently. Even after the gate voltage is put into an OFF state, the ripple voltage is kept on attenuated. Therefore, the drain voltage (pixel electrode potential), which is influenced by the CS voltage, is diverged from the level that the drain voltage is to have, due to the remaining ripple voltage Vα.
In the same pixel row, the ripple voltages having opposite polarities act to counteract each other. However, the ripple voltages having the same polarity are superimposed on each other. Therefore, as shown in FIG. 17 and FIG. 18, in the case where the polarities of all the voltages applied to the pixels of the same color in the same pixel row are the same, when the window pattern is displayed with a single color, a horizontal shadow is generated.
Patent Document 3 discloses a technology for preventing the generation of the horizontal shadow. FIG. 20 shows a liquid crystal display device 1000 disclosed in Patent Document 3.
As shown in FIG. 20, the liquid crystal display device 1000 includes a liquid crystal display panel 1001 including color display pixels CP each including a red pixel R, a green pixel G, a blue pixel B and a white pixel W, and a source driver 1003 for supplying a display signal to a plurality of signal lines 1013 provided in the liquid crystal display panel 1001.
The source driver 1003 includes a plurality of independent drivers 1003a which correspond to the plurality of signal lines 1013 in a one-to-one manner. The plurality of independent drivers 1003a are arrayed in a row direction, and two independent drivers 1003a adjacent to each other output gray scale voltages of opposite polarities to each other.
In the liquid crystal display device 1000 shown in FIG. 20, the arraying order of a part of the signal lines 13 is inverted outside a display area. For example, the fifth signal line 1013 and the sixth signal line 1013 from the left in the figure are provided to intersect each other outside the display area in the state where an insulating film is interposed therebetween. As a result, the fifth signal line 1013 is connected to the sixth independent driver 1003a, and the sixth signal line 1013 is connected to the fifth independent driver 1003a. Owing to such a structure, in the liquid crystal display device 1000, the pixels of the same color in the two color display pixels CP adjacent to each other in the row direction are supplied with gray scale voltages of opposite polarities to each other. For this reason, the polarities of the voltages applied to the pixels of the same color arrayed in the row direction are not the same, and thus the generation of a horizontal shadow can be prevented.